This invention relates to apparatus for increasing the effective length of the boom of a crane, and more particularly to improved methods of and apparatus for erecting boom extensions and storing them when not in use.
It has been proposed to provide two extension units for a multi-section telescopic boom which can be used if desired or can be stowed in brackets on the boom when not in use. In the arrangement disclosed in U.S. Pat. No. 4,091,936 one unit, which is identified as a fly and is stored alongside the boom, is arranged to be pivotally connected to the forward end of the outermost boom section and swung around to a position of alignment with the boom. The second unit, called a jib, underlies the fly when both the fly and the jib are in stowed position on brackets on the boom and is arranged to be pivotally connected, if desired, to the fly so that it will swing outwardly with the fly when the fly is swung to the aligned position, and then can be swung downwardly and outwardly relative to the fly in a generally vertical plane until it too reaches aligned position with the boom. Various other arrangements for increasing the effective length of a crane boom and for storing boom extension units are disclosed in U.S. Pat. Nos. 3,366,250; 3,732,988; 4,106,631; 4,141,455; and 4,155,464.
Since this type of equipment is very heavy and difficult to maneuver, the storing of the extension units and the moving of them relative to each other is particularly difficult and improved mechanisms in these areas are desirable.
In addition, regulatory codes require that a backstop device be provided between a fly and jib, when erected, to prevent the jib from pivoting upwardly relative to the fly in excess of 5.degree. above the longitudinal axis of the fly and boom. Heretofore, jibs that have been supported on flys at the ends of booms by pendant lines and mast, and have required expensive and somewhat cumbersome backstop devices to prevent the jib from rebounding or swinging upwardly in excess of 5.degree. above the centerline of the boom when the boom is raised to a near vertical position. Backstops in the form of tubular telescopic struts which are connected at an angle between the fly and the upper end of the mast are known. These struts are arranged to bottom out when the maximum permissible upward swing of the jib has been reached.